This invention relates generally to an electronic switching circuit and more specifically to a switching or gating circuit which is to be utilized with an image intensifier tube, a CRT type device which produces a visual image on a phosphorous screen.
In the past, image intensifier tubes were controlled in an on or off posture by various gating circuits which switched the polarity of the tube focusing electrodes from a plus potential (about 350 volts) to a minus potential (about -1300 volts) with respect to the tube cathode. This required a switching circit capable of handling voltage in the magnitude of 1600 to 1800 volts which can be switched using a high number of high number of high voltage solid state controlled rectifiers connected in series with appropriate voltage equalizers. Utilizing the focusing anode for switching the tube image on and off creates a defocusing affect on the image because of the time constant required for the switching circuit to reach equilibrium to produce the required biasing voltage between the cathode and the focusing electrodes. The defocusing affect on the image is detrimental to the image quality, especially if the image is to be recorded on a photographic sensitive film. It is oftentimes desirous to expose the image on the tube over a period of time to sensitive photographic film such that any defocusing or scintillations when the tube is biased off could make the film exposure very distorted. Scintillations occur while the tube image is biased off from spurious electrode flow resulting from high voltage present between the tube cathode and anode which cause uncontrollable random bright spots on the tube phosperous surface. Also when the tube is in the off position, the tube image face cannot be made completely dark. Thus overall the use of image tube focusing electrodes for switching tube image on and off has in the past created deleterious affects if the tube is used in conjunction with photographic sensitive film.
The instant invention eliminates the problems of the prior art by providing a gating circuit which controls the voltage across the cathode and anode of the image intensifier tube, thus switching the device completely on or completely off while eliminating the defocusing effect and scintillations found in the prior art.